2h Frost relies too much on Obliterate for their output. 2x Necrotics might be more overall than an Obliterate hit - but not much more. And it's less than an Obliterate crit. And you don't get to see the big numbers.

Dual-wield frost can and should use it on healers - but, one, there's not a lot of us, and, two, you get people who refuse to use it because it doesn't benefit from Threat of Thessarian.

It doesn't make it not worth using (it is - these days, it can give you 2x if not 3x more value for your rune than a single Howling Blast, if you count both the hit and the total absorb value), but, uh, for some it's just the principle of the thing. They see that it doesn't get that off-hand hit, and they boycott it. Like a hunger strike.

For frost wile it has its place in setting up kills. over all starts to hurt frost. with frost not having reaping they only get 2 necros which use to add up to a total of 60k aborbe. not to big but more then enough to help land that kill. with the most recent nerf/buff it absorbed for 35-40k health still useful but harder to set up the kill. so most would rather hit them with a big oblit then a weakened Necro.

large random oblit crits tend to make healers and people in general panic and force cds before the dk is actually bursting. backloaded necrotics can help but they cant be stacked as effectively in frost.